Process for the electrographic recording of charge images

ABSTRACT

In a process for the production of electrostatic images on insulating recording material the recording material is moved over a contact electrode and over the point of contact with this electrode, the charge image is recorded by a suitable arrangement of character writing electrodes, while the contact electrode is wetted with a conductive liquid at the point of contact with the recording material.

United States Patent Simm 1 Feb. 18, 1975 [54] PROCESS FOR THE ELECTROGRAPHIC 1,328,403 1/1920 Shaw 317/2 R RECORDING OF CHARGE IMAGES 2,935,418 5/1960 Bcrthold et a1 317/262 A 3,147,415 9/1964 Olrphant 317/262 A [75] Invento a e 8 Leverkusen, 3,546,545 12/1970 S2110 ct al 317/262 A Germany 3,636,408 l/1972 Shuman 1 317/2 R 9 d d: 7 6 [73] Ass1gnee: Agia-Gevaert Aktiengesellschaft, 3 751715 8/1 73 E S 31 /2 2 A Leverkusen, Germany Primary Examiner-L. T. Hix [22] Fled: June 1973 Attorney, Agent, or FirmConnolly and Hutz [21] Appl. No.: 373,681

[57] ABSTRACT [30] Forelgn Apphcauon Prlomy Data In a process 'for the production of electrostatic images June 28, 1972 Germany 2231530 on insulating recording material the recording mate rial is moved over a contact electrode and over the [52] US. Cl 317/262 A point of Contact with this electrode, the Charge image [51] 111i. Cl H05 is recorded by a Suitable arrangement of character [58] F'eld of Seal-chm 317/2 262 262 R writing electrodes, while the contact electrode is wetted with a conductive liquid at the point of contact [56] References C'ted with the recording material.

UNITED STATES PATENTS 1,163,656 12/1915 Hadaway, Jr. 317/2 R 19 5 Drawmg F'gures PATENfl-ju FEB 1 8 I975 SHEET 10F 2 FIG] PATENIEU H35 1 81975 SHEET 2 BF 2 FIGS PROCESS FOR THE ELECTROGRAPHIC RECORDING OF CHARGE IMAGES This invention relates to an electrographic process for recording charge images on foils made of highly insulating material which may, if desired, be transparent.

It is known that charge images can be recorded on paper which can be electrostatically charged. The recording material generally consists of paper which has been coated with two or three coats consisting of a conductive carrier with a thin insulating layer or of a carrier of medium conductivity, a highly conductive intermediate layer, and an insulating layer arranged above this. Such paper is generally very suitable for recording letters or numbers which are printed on them directly by the known electrostatic printing process in which electric impulses are applied with various electrodes. The known processes are carried out using e.g., pin electrodes in series or as a matrix arrangement and placed on the recording material or close to the surface of the carrier for the recording.

The recording may be made using, for example, an electron beam tube from which the charge of the electron beam is transmitted to the recording paper by pin electrodes arranged in a series in the face plate of the tube. The recording paper is moved past the front of the electrodes whilst very close to them (only a few um away from them) while the back of the paper is in contact with a counterelectrode. The electron beam of the tube can be controlled by a video signal according to the position and intensity so that a charge image is produced on the paper.

in other known processes which allow for substantial control of the intensity of the charging current, the charging current is produced by a corona discharge which is operated continuously without interruption and from which partial currents are removed through slit diaphragms or apertured diaphragms. Image-wise control of these partial currents is effected by electric signals which are transmitted to the apertures of the diaphragms by suitable control electrodes.

In order to produce two-tone images which have no intermediate tones, the fibrous structure of the paper used as carrier for the insulating layer is generally sufficient as regards homogeneity and surface structure. A certain roughness on the surface is even desirable since a slight distance between the electrode and surface of the paper is necessary to ensure discharge on the recording electrodes.

If imagewise charging is effected by a constant charging current and if the charge is required to be uniformly distributed over the surface of the paper and its density is required to be graded in half tones, then the quality of the recording material must satisy quite different requirements. Under these conditions, the fibrous structure of the paper is a very serious disadvantage. For a better quality image it is therefore necessary to use a These requirements are largely fulfilled by the ordinary commercial plastics foils such as foils of polyterephthalic acid esters or polycarbonate foils. These foils have the high resistance necessary to produce stable charge images as well as an electrical insulation value sufficient to enable them to be charged to high potentials which is advantageous for the development of the image.

The use of such plastic foils as recording carriers in electrographic recording processes does, however, entail the following difficulties:

During the charging process, the recording carrier must be brought into intimate contact with the surface of the counterelectrode in order that a homogeneous surface charge can be obtained. Such contact is achieved only very incompletely if the recording carrier is placed on a flat metal plate. Discharges therefore occur in the layers of air enclosed between recording carrier and support during the imagewise charging of the recording carrier so that variations in the charge density and hence faults in the recording of the image result.

In addition, the plastics foil used as recording carrier retains electric charges both on the image side and on the back after charging. The back is liable to become charged by contact with the support even when the image side is not charged. These charges cannot be controlled and seriously interfere with the development of the charge image on the other side of the foil unless the charges on the back of the foil can be completely neutralised in the development process by neat contact with a conductive support.

It is an object of this invention to develop a process by which highly insulating foils can be used as recording carriers for electrographic recordings without requiring any treatment with conductivity dressings.

A process has now been found by which electrographic image recordings can be made on smooth, highly insulated carrier material, e.g., plastics foils, in which the recording carrier is moved over a contact electrode, and over the point of contact with this elec- I trode, the charge image is recorded on the other side of the carrier by a suitable arrangement of character writing electrodes, the process being characterised in that the contact electrode is wetted with a conductive liquid at the point of contact with the carrier.

The process according to this invention also consists in that a line contact is established between the recording carrier and the wetted contact electrode by means of the fact that during the imagewise charging, the recording carrier is pulled over one edge of the contact electrode and is charged only along the line of this edge.

Compared with a counterelectrode consisting of a plane surface, this edge in any case provides better contact for the recording carrier because the formation of gaps between the carrier and the contact electrode is more efficiently suppressed along a line of contact with pressure but the distinct improvement in the quality of the recording which renders the process suitable for half-tone recording is achieved only when in addition in accordance with the invention the edge is wetted with a conductive liquid.

According to the process of the invention, therefore, the electrode on which the recording carrier is placed is wetted with a liquid which penetrates the gaps underneath the recording carrier and thus ensures better contact with the support.

The invention will now be described in detail with reference to the schematic representation in the drawings.

FIG. 1 represents an embodiment of the electrode used as support for the recording carrier during recording of the charge image. The electrode formsan edge which is wetted with a liquid from the inside,

FIG. 2 shows an electrode arrangement for the charge recording,

FIG. 3 shows another embodiment of the electrode which is adapted to be wetted,

FIG. 4 shows another embodiment of the electrode which is wetted, and

FIG. 5 is a schematic representation of a recording apparatus in which this invention is embodied.

According to FIGS. 1 and 2, the recording carrier 1, e.g., polyterephthalic acid ester foil ,u in thickness, is stretched over the electrode 2 with support edge 3 and moved in the direction of the arrow. The electrode encloses a cavity 4 which is open to the support edge through a flat channel 5. This cavity and channel are completely filled with a liquid, e.g., water, the opening being covered by the foil which is stretched over the edge. The cavity of electrode 2 communicates by way of a connecting tube 6 with a container 8 for liquid which is closed by the flexible membrane 7. The liquid in the container 8 can be used for replacing the small quantities of liquid used up. The membrane is then readjusted by the set screw 9. The membrane 7 which closes the container and the action of the external air pressure prevent excessive quantities of liquid from being removed by the moving foil. Stable operating conditions are thereby obtained.

Liquids suitable for wetting the edge 3 should not have any deleterious effect on the recording material, for example they should not cause it to swell, dissolve or the like. Moreover, the liquids should have an electric conductivity greater than 10 Ohm cm, preferably greater than 10 Ohm cm.

Apart from water, which has already been mentioned, the liquid used may be benzyl alcohol, diethylene glycol monoethylether or ethyl glycol. Readily evaporating organic liquids are also suitable, e.g., methyl alcohol, ethyl alcohol, ethylene chloride or ethyl acetate.

The following are examples of foils of polymer synthetic resins which are suitable for use as recording carriers: Cellulose triacetate, cellulose acetobutyrate, polyethylene, polystyrene, polyvinyl chloride, polyisobutylene and polytetrafluoroalkylene.

The thickness of the foil is preferably between 10 and 60 um. Foils thinner than this are technically difficult to produce while thicker foils have the disadvantage that the capacity per unit surface area and hence the density of charge which can be achieved become too small.

The specific electric resistance of suitable foils is in the range of 10 to 10 Ohm cm.

A suitable electrode arrangement for the electrographic recording of foil material using electrode 2 is shown in FIG. 2. A partial current from a corona discharge produced at the discharge electrode 10 passes through the aperture of a split diaphragm composed of the parts 11, 12, 13 and 14 to reach the recording carrier l where it produces a charge exactly over the line of contact of the carrier 1 with the electrode edge 3. According to the description of the recording process given in US. Pat. No. 3,594,162 or in Deutsche Offenlegungsschrift 2,148,001 the partial discharge current passing through the diaphragm composed of four flat electrodes 11, 12, 13 and 14 separated by insulating interlayer 24 can be controlled by electrical image signals. For this purpose, at least one of the electrodes is subdivided into a plurality of conductive strips along which the signal voltage U is conducted. The electrode 2 is biased with a positive bias voltage with regard to the diaphragm from a voltage source 23 while electrode 10 is connected with the negative output of voltage source 20 and therefore repels the negative charge carrier formed in the corona.

In FIG. 3, channel 5 is filled with absorbent filling material 15. The capillary forces thereby set up draw the liquid to the recording carrier 1.

Another form .of wettable electrode is shown in FIG. 4. The electrode body is made of solid material and has a sharp edge 17. The recording carrier 1 is moistened by pressing the film against a moistener 18 of soft, absorbent material soaked with liquid before it is brought into contact with the edge of the electrode.

The process according to the invention will now be explained in more detail with the aid of the following examples.

EXAMPLE 1 Using the arrangement illustrated in FIG. 5, a polyterephthalic acid ester film 1 which has a thickness of 10 um and a width of mm is pulled over the edge 3 of electrode 2 at the rate of 30 cm/s. The electrode 2 encloses the cavity 4 which is open to the top through the channel 5 which has a width of 0.1 mm. This electrode isconnected to the positive terminal ofa voltage source The slot shaped aperture of a diaphragm which consists substantially of metal sheets 11 to 14 fixed to an insulating interlayer 24 is situated at a distance of 0.5 mm above the edge 3. The parts of this diaphragm are clamped into a holder 19. The metal sheets 11 to 14 are insulated from the holder by additional interlayers 26 and electrically connected as shown in FIG. 2.

The discharge electrode 10 is arranged 100 mm above the gap and connected to the voltage source 20. The discharge space is enclosed in an insulating housing 21.

Electric charging of the polyterephthalic acid ester film 1 is carried out in a nitrogen atmosphere which is produced by a stream of nitrogen (400 l/h) flowing in the direction of the arrows. To produce the charging current, a negative voltage of 25 kV is applied to the discharge electrode 10 and a positive voltage of 2 kV to electrode 2. The control voltage on the sheet 11 is adjusted toa constant value, e.g., to U 0 or l0 V at which the charging current is so high that distinct blackening is observed after development of the charged film. The cavity 4 of electrode 2 is at first not filled with a contact liquid.

Consequently, the surface of the film becomes charged with a two-dimensional charge which after development with one of the known electrophoretic developers assumes a black colour in which the structure of lighter and darker stripes are recognizable. If the control voltage was so adjusted that only a black of medium intensity could be produced, then the stripes are particularly conspicuous and disturbing. Suitable electrophoretic developers have been described by Metcalfe and Wright in 1. Scientific lnstr. 33, 194 (1956) or in US. Pat. No. 2,907,674.

EXAMPLE 2 A second blackening test is carried out in which the cavity 4 and channel 5 of electrode 2 are filled with conductive water. The experimental conditions are otherwise the same as in Example 1.

As a result, a homogeneously blackened surface is obtained on which no strips can be seen. The blackness obtained is denser than the medium density blackness obtained in Example 1.

EXAMPLE 3 Other blackening tests are carried out after the water in electrode 2 has been replaced by benzyl alcohol. Uniformly blackened surfaces with further improvement in the density are obtained.

The process according to the invention thus enables ordinary commercial foils to be used as recording carriers for electrographic image recording instead of expensive multilayer paper. By using the apparatus described in the examples, imagewise charges which when developed are distinguished from two-tone recordings by a plurality of possible density values are ob tained.

What we claim is:

1. A proces for the production of electric charge distribution on a smooth insulating web having a specific resistance of from about to 10 Ohm cm. which moves relative to an electrode assembly comprising a first electrode which can apply varying electric charges to one surface of the web and a second electrode which contacts the other surface of the web at least at a position adjacent to the area in which the first electrode can apply charges characterized by the steps of provid ing a contact edge on the second electrode, traversing the insulating web in firm contact over the edge, providing a source ofa conductive liquid having an electrical conductivity greater than about 10 Ohm cm adjacent the edge, and contacting the web with a source of liquid and the edge to dispose the conductive liquid inbetween the web and the contacted edge whereby intimate electrical contact is obtained between the web and the second electrode.

2. A process as claimed in claim 1 in which the first electrode can apply varying charges along a line transverse to the direction of motion of the web.

3. A process as claimed in claim 2 in which the second electrode comprises a substantial straight edge over which the web is drawn.

4. A process as claimed in claim 3 in which part of the liquid is contained inside the second electrode and is conducted to the edge by a channel, or channels.

5. A process as claimed in claim 4 in which the channel or channels contain a wick or wicks which'contact the web at the point of contact of the web with the second electrode.

6. A process as claimed in claim 3 in which the other surface of the web is moistened before it reaches the point of contact with the second electrode.

7. A process as claimed in claim 6 in which the web is moistened with a wick or pad which is kept moist with the conductive liquid.

8. A process as claimed in claim 1 in which the conductive liquid used is water.

9. A process as claimed in claim 1 in which the web is a web of polycarbonate or polyterephthalate plastics.

10. A process as claimed in claim 1 in which the web is 10 to 60pm thick.

11. A process as set forth in claim 1 wherein the web is a plastic foil.

12. A process as set forth in claim 11 wherein the first electrode applies a corona discharge.

13. A process as set forth in claim 11 wherein the contact with the source of conductive liquid is made immediately before the web contacts the edge.

14. A process as set forth in claim 11 wherein the contact with the source of conductive liquid is made immediately at the edge.

15. A contact electrode for helping produce an electric charge distribution on an insulating web which moves in contact relative to the electrode comprising a body, a relatively thin edge on the body contacting the web, and a channel withinthe thin edge for supplying a conductive liquid to the area of the web contacted by the thin edge.

16. An electrode as claimed in claim 15, wherein a conduit is provided within the electrode for supplying the conductive liquid to the channel.

17. An electrode as claimed in claim 15, wherein a wick connects the channel with the conduit and extends to the edge of the channel.

18. An electrode as claimed in claim 15, wherein an absorbent pad soaked with the conductive liquid is provided on a side of the electrode in a position wherein it is contacted by the web before the web passes over the edge of the electrode.

19. An electrode as claimed in claim 15, wherein a discharge electrode is spaced from the contact electrode for providing charges to the side of the web remote from the contact electrode, an apertured diaphragm is disposed between the discharge electrode and the contact electrode and adjacent the contact electrode, and electrical potential means connected to the apertured diaphragm for controlling the charging of the surface of the web. 

1. A proces for the production of electric charge distribution on a smooth insulating web having a sPecific resistance of from about 1012 to 1019 Ohm cm. which moves relative to an electrode assembly comprising a first electrode which can apply varying electric charges to one surface of the web and a second electrode which contacts the other surface of the web at least at a position adjacent to the area in which the first electrode can apply charges characterized by the steps of providing a contact edge on the second electrode, traversing the insulating web in firm contact over the edge, providing a source of a conductive liquid having an electrical conductivity greater than about 10 9 Ohm 1cm 1 adjacent the edge, and contacting the web with a source of liquid and the edge to dispose the conductive liquid inbetween the web and the contacted edge whereby intimate electrical contact is obtained between the web and the second electrode.
 2. A process as claimed in claim 1 in which the first electrode can apply varying charges along a line transverse to the direction of motion of the web.
 3. A process as claimed in claim 2 in which the second electrode comprises a substantial straight edge over which the web is drawn.
 4. A process as claimed in claim 3 in which part of the liquid is contained inside the second electrode and is conducted to the edge by a channel, or channels.
 5. A process as claimed in claim 4 in which the channel or channels contain a wick or wicks which contact the web at the point of contact of the web with the second electrode.
 6. A process as claimed in claim 3 in which the other surface of the web is moistened before it reaches the point of contact with the second electrode.
 7. A process as claimed in claim 6 in which the web is moistened with a wick or pad which is kept moist with the conductive liquid.
 8. A process as claimed in claim 1 in which the conductive liquid used is water.
 9. A process as claimed in claim 1 in which the web is a web of polycarbonate or polyterephthalate plastics.
 10. A process as claimed in claim 1 in which the web is 10 to 60 Mu m thick. .
 11. A process as set forth in claim 1 wherein the web is a plastic foil.
 12. A process as set forth in claim 11 wherein the first electrode applies a corona discharge.
 13. A process as set forth in claim 11 wherein the contact with the source of conductive liquid is made immediately before the web contacts the edge.
 14. A process as set forth in claim 11 wherein the contact with the source of conductive liquid is made immediately at the edge.
 15. A contact electrode for helping produce an electric charge distribution on an insulating web which moves in contact relative to the electrode comprising a body, a relatively thin edge on the body contacting the web, and a channel within the thin edge for supplying a conductive liquid to the area of the web contacted by the thin edge.
 16. An electrode as claimed in claim 15, wherein a conduit is provided within the electrode for supplying the conductive liquid to the channel.
 17. An electrode as claimed in claim 15, wherein a wick connects the channel with the conduit and extends to the edge of the channel.
 18. An electrode as claimed in claim 15, wherein an absorbent pad soaked with the conductive liquid is provided on a side of the electrode in a position wherein it is contacted by the web before the web passes over the edge of the electrode.
 19. An electrode as claimed in claim 15, wherein a discharge electrode is spaced from the contact electrode for providing charges to the side of the web remote from the contact electrode, an apertured diaphragm is disposed between the discharge electrode and the contact electrode and adjacent the contact electrode, and electrical potential means connected to the apertured diaphragm for controlling the charging of the surface of the web. 